The present invention relates generally to fire alarm systems and deals more particularly with an improved method and apparatus for determining the location of a high temperature zone along the length of the fire detecting cable.
It is known in the art of fire alarm systems to utilize a special electrical cable typically made of two conductors which produce a short circuit between one another wherever the insulation of the conductors melts as a result of the cable being heated or overheated, for example by a fire, at some point along the length of the cable. Various means are known for producing a short circuit between the conductors of such a cable, for example, semi-conductor devices designed for such operation are placed at spaced intervals along the cable and become conductive when exposed to heat such as heat produced during a fire condition. The location of the short circuit is then determined by measuring the resistance between the ends of the conductors at one end of the cable. Since the resistance per unit length of the conductors is generally known, the value of the measured resistance may be used to provide a determination of the distance along the cable to the location of the short circuit. One such typical prior art method and apparatus is shown as an electrical schematic representation in FIG. 1.
The accuracy of the determined location of the short circuit using the above known method and associated apparatus is generally poor because the resistance of the conductors varies with changes in ambient temperature and further because the short circuit itself may present an electrical resistance which contributes to the overall resistance measurement thus leading to an erroneous determination of the location of a high temperature zone.
Furthermore, the above known method and associated apparatus is unsuitable for use with a new class of fire detecting or temperature sensing cable which does not physically short circuit between conductors but rather has insulation which has a composition which establishes a conduction path between the conductors at elevated temperatures. Such temperature sensing cable is disclosed, for example, in U.S. Pat. No. 5,185,594.
It is an aim of the present invention therefore to overcome the above limitations by providing an improved method and associated apparatus to determine the distance from one end of a fire detecting or temperature sensing cable to the point of the short circuit established between conductors at elevated temperatures with greater accuracy than that provided by resistance measurement or time domain reflectometry.
It is a further aim of the present invention to maintain the improved accuracy in the determination of the distance from one end of the fire detecting cable to the point of the elevated temperature along the cable regardless of the resistance added by the short circuit producing means itself, and regardless of any resistance variation in the conductors of the fire detecting or temperature sensing cable due to ambient temperature changes.
It is a yet further aim of the present invention to provide a method and related apparatus which operates with fire detecting or temperature sensing cable having insulation which becomes electrically conductive at high temperatures to permit a greater degree of sensitivity and accordingly, the ability to detect fires or overheat locations earlier than might otherwise be possible using known methods and associated apparatus.